Some data transmission systems used in data communication apparatuses adopt bus standards, e.g., USB (Universal Serial Bus), IEEE (Institute of Electrical and Electronics Engineers) 1394, etc. Such transmission systems include a plurality of signal lines.
In general, a data transmission system is provided for a plurality of apparatuses communicating data with each other, and is used as a system for data communication among them. For example, a data transmission system is used for data communication between a computer operable as a host device, and an information storage terminal connected to the computer, such as an SD card, a USB memory, or an external hard disk. The capacity of such an information storage terminal is increasing year by year, which in turn allows to handle large amounts of data.
In order to comfortably use a large-capacity information storage terminal, it is necessary to significantly increase data transmission rate. In addition, various electrical appliances are being diversified, and they may work in cooperation with each other. Hence, it is possible to suppose a case in which various electrical appliances each including information storage medium perform data transmission to each other. Also in such a case, a high-speed data transmission scheme for the respective devices is required.
There is a transmission scheme for high-speed data transmission, called a “differential transmission scheme”. According to the differential transmission scheme, a pair of signals of opposite phases are transmitted over a pair of transmission lines, respectively, thus generating a signal from a difference between the pair of signals. According to the differential transmission scheme, since the signal amplitudes can be reduced, the signal frequency can be made faster. In addition, since a signal is generated from a difference between the pair of signals of opposite phases, the differential transmission scheme is characterized by being highly resistant to noise (common mode noise). In addition, since the pair of signals of opposite phases are transmitted over the pair of transmission lines, magnetic fields generated from the respective transmission lines are cancelled out, and thus, the differential transmission scheme is characterized in that unwanted radiation noise is less likely to be generated.
In the case of a data transmission system using the differential transmission scheme, in general, a termination resistor is connected to respective signal lines in order to prevent signal reflections, the termination resistor having a resistance value that match a differential impedance of the signal lines.
Meanwhile, in data transmission between a plurality of apparatuses, when a plurality of signal lines provided between the apparatuses are used as signal lines for communication in one direction at a time, there is a need for a means of transmitting, during data transmission, an interrupt signal such as a control signal for requesting to stop the transmission.
As a data transmission method that allows to transmit an interrupt signal during data transmission, there is a transmission method disclosed in Patent Document 1. According to Patent Document 1, when data is transmitted from a transmitting apparatus to a receiving apparatus through a single transmission path, the transmitting apparatus transmits packets such that an interrupt gate period (time slot) is provided between packets transmitted. The receiving apparatus can transmit an interrupt signal (e.g., a signal for requesting to stop transmission) to the transmitting apparatus during the interrupt gate period.
However, according to the transmission method of Patent Document 1, it is necessary to allocate an interrupt gate period having a bit length equal to or greater than that of an interrupt signal, and it is also necessary to provide an interrupt gate period between packets even when actually requiring no interrupt. Hence, it creates a problem of degradation in the transmission efficiency of the transmitting apparatus.
As a transmission method that solves the problem, there is a scheme disclosed in Patent Document 2. According to Patent Document 2, a change in the termination resistance value at a receiving apparatus results in a change in the amplitudes of differential signals, and the latter change is used as an interrupt signal. With reference to FIG. 20, the signal transmission method disclosed in Patent Document 2 will be described.
An apparatus A is a transmitting apparatus, and an apparatus B is a receiving apparatus. Referring to FIG. 20, the receiving apparatus B is provided with a resistor having a variable resistance value, as a termination resistor 73a for a signal line D0. The apparatus B is further provided with a circuit 82 for changing the resistance value of the termination resistor 73a. The apparatus A is provided with a circuit 84 for detecting amplitudes of differential signals in the transmitting apparatus of the signal line D0.
Each of the apparatuses A and B is operable as either of a transmitting apparatus and a receiving apparatus. Hence, the apparatus A is provided with a termination resistor 72a having a variable resistance value, and a circuit 83 for changing the resistance value of the termination resistor 72a, and the apparatus B is provided with a circuit 85 for detecting amplitudes of differential signals.
In order to transmit an interrupt signal from the receiving apparatus to the transmitting apparatus in this configuration, the termination resistance value at the receiving apparatus (B/A) is decreased. The decrease in the termination resistance value results in a change in the amplitudes of differential signals. The changes in the differential signals can be detected by the transmitting apparatus (A/B). The change in the amplitudes of the differential signals is used as an interrupt signal. For example, the configuration can be such that, when the transmitting apparatus (A/B) has detected changes in the amplitudes of differential signals, the transmitting apparatus (A/B) stops data transmission and waits for a response from the receiving apparatus (B/A). In addition, the beginning (header) and end (footer) of a packet are usually assigned a predetermined signal string portion. The receiving apparatus (B/A) changes the amplitudes of the differential signals when receiving such a predetermined signal string of a packet.
Hence, the resistance changing circuit 82 for changing the termination resistance value to change the amplitudes of differential signals, and the amplitude detection circuit 84 for detecting amplitudes of differential signals are provided along the signal line D0. Thus, as described above, the amplitudes of differential signals can be changed when receiving a predetermined signal string of a packet. Therefore, according to this method, it is not necessary to provide any extra interrupt gate period between packets, and thus, no degradation in transmission efficiency occurs when an interrupt signal is transmitted.
In addition, the termination resistance value at the receiving apparatus is usually matched with an intrinsic impedance of a corresponding one of the signal line D0. This prevents signal reflections in a transmission path, thus maintaining signal quality and preventing the generation of unwanted radiation noise.